Fix use-after-free issue in libpagestore (#4239)

## Describe your changes

`pageserver_disconnect()` calls `PQfinish()` which deallocates resources
on the connection structure. `PQerrorMessage()` hands back a pointer to
an allocated resource. Duplicate the error message prior to calling
`pageserver_disconnect()`.

## Issue ticket number and link

Fixes https://github.com/neondatabase/neon/issues/4214

## Checklist before requesting a review

- [x] I have performed a self-review of my code.
- [x] If it is a core feature, I have added thorough tests.
- [x] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [x] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [x] Do not forget to reformat commit message to not include the above
checklist

pgxn/neon/libpagestore.c

@@ -367,7 +367,7 @@ pageserver_flush(void)
 	}
 	else if (PQflush(pageserver_conn))
 	{
-		char	   *msg = PQerrorMessage(pageserver_conn);
+		char	   *msg = pchomp(PQerrorMessage(pageserver_conn));
 
 		pageserver_disconnect();
 		neon_log(ERROR, "failed to flush page requests: %s", msg);